Polarization multiplexing/demultiplexing is an important function in a high-speed coherent optical transmission system. One technique used to perform polarization multiplexing/demultiplexing on an optical waveguide is to use a Mach-Zehnder (MZ) interferometer based polarization multiplexer/demultiplexer. In order to provide polarization multiplexing/demultiplexing, the MZ interferometer is designed to create a π phase difference between two orthogonal polarization states of light as the light propagates through optical waveguides of the arms of the MZ interferometer. The π phase difference is introduced by trenches formed near an optical waveguide of one arm of the MZ interferometer. The trenches serve to reduce compressive stress on the optical waveguide. This change in compressive stress affects a waveguide propagation constant for the two orthogonal polarizations differently. Here, by knowing the difference of the waveguide propagation constant between the two orthogonal polarizations, a trench length can be selected that produces π phase birefringence such that a π phase difference is introduced to the two orthogonal polarizations when passing through the optical waveguides of the MZ interferometer.